The present invention generally relates to controlling the temperature of a workpiece. More particularly, the present invention relates to a thermally controlled rotatable wafer chuck to directly control the temperature of a semiconductor wafer during spin dispensing of a process liquid onto the wafer.
Integrated circuits are typically constructed by depositing a series of individual layers of predetermined materials on a wafer shaped semiconductor substrate, or "wafer". The individual layers of the integrated circuit are in turn produced by a series of manufacturing steps. For example, in forming an individual circuit layer on a wafer containing a previously formed circuit layer, an oxide, such as silicon dioxide, is deposited over the previously formed circuit layer to provide an insulating layer for the circuit. A pattern for the next circuit layer is then formed on the wafer using a radiation alterable material, known as photoresist.
Photoresist materials are generally composed of a mixture of organic resins, sensitizers and solvents. Sensitizers are compounds, such as diazonaphthaquinones, that undergo a chemical change upon exposure to radiant energy, such as visible and ultraviolet light. The irradiated sensitizer material has different salvation characteristics with respect to various solvents than the nonirradiated material allowing for selective removal of the photoresist. Resins are used to provide mechanical strength to the photoresist and the solvents serve to lower the viscosity of the photoresist so that it can be uniformly applied to the surface of the wafers.
After a photoresist layer is applied to the wafer surface, the solvents are evaporated and the photoresist layer is hardened, usually by heat treating the wafer. The photoresist layer is then selectively irradiated through the use of a radiation opaque mask. The mask contains transparent portions that define the pattern for the next circuit layer. The mask is placed over the photoresist layer and the photoresist covered by the transparent portion is irradiated. The mask is removed and the photoresist layer is exposed to a process liquid, known as developer. The developer selectively solubilizes and removes either the irradiated or the nonirradiated photoresist exposing portions of the underlying insulating layer.
The exposed portions of the insulating layer can be selectively removed using an etchant to expose corresponding sections of the underlying circuit layer. In this process, the photoresist should be more resistant to the etchant than the insulating layer to limit the attack of the etchant to only the exposed portions of the insulating layer. Alternatively, the exposed underlying layer(s) can be implanted with ions which do not penetrate the photoresist layer thereby selectively penetrating only those portions of the underlying layer not covered by the photoresist. The remaining photoresist is then stripped using either a solvent, or a strong oxidizer in the form of a liquid or a gas in the plasma state. The next layer is then deposited and the process is repeated until fabrication of the semiconductor device is complete.
Process liquids, such as photoresist, developer, etchants and solvents, are typically applied to the wafer using a spin dispensing technique in which the liquid is dispensed on the surface of the wafer as the wafer is spun on a rotating chuck. The spinning of the wafer distributes the liquid over the surface of the material and exerts a shearing force that separates some of the liquid from the wafer leaving a thin liquid layer on the surface of the wafer. A thin uniform layer of process liquid is important to provide for uniform action of the process liquid on the wafer surface, such as with developer, or to provide a suitable layer for the formation of subsequent circuit layer, as with photoresist.
The uniformity of the process liquid on the wafer depends on a number of factors, one of which is the temperature of the process liquid on the wafer. Generally, as the temperature of the wafer is increased, the process liquid dispensed onto the wafer will be more easily distributed over the surface due to decreasing viscosities of most process liquids with increasing temperature. However, for process liquids, such as photoresist, that have both volatile and nonvolatile components the increased temperature will cause accelerated evaporation of the volatile components from the process liquid. The loss of the volatile components increases the viscosity of the remaining process liquid, thereby making it more difficult to distribute the process liquid over the surface of the wafer. Whereas, lowering the temperature of the wafers will generally lower the viscosity of the process liquid making it more difficult to distribute the process liquid over the wafer surface. Although, in the case of photoresist, the lower temperature will reduce the amount of volatile component evaporation and resulting viscosity increases during the process. The nonuniform distribution of the process liquid resulting from temperature variations across the wafer contributes to defects in the coating layers which reduce the overall yield of properly performing chips from the wafer.
One attempt made in the prior art to provide for a more uniform distribution focused on minimizing the heat transferred to the wafer through the chuck from the spin motor, as disclosed in U.S. Pat. No. 5,343,938 issued to Schmidt. The Schmidt patent discloses an insulated wafer chuck for use in spin dispensing. The chuck contains an interior chamber that is either evacuated or filled with gas having a low thermal conductivity. The chamber serves to thermally isolate the wafer chuck from the spindle and spin motor so that the temperature of the chuck and the wafer are controlled by the surrounding environment. A problem with the prior art, including the Schmidt patent, is that the temperature of the wafer and the chuck are passively controlled by the surrounding environment in the spin dispensing apparatus. As a result, the wafer is subject to the temperature variations that occur in the surrounding environment, which generally must be controlled very precisely to ensure that the wafer is at the correct temperature for optimal uniformity of the dispense liquid. In the prior art, the environmental temperature in the processing chamber is oftentimes different than the temperature at which the wafer is stored. Therefore, an extra step of preconditioning the wafer in a special module is required prior to loading the wafer in the processing chamber. These problems all serve to increase the variability and complexity of the liquid spin dispensing process.
Thus, it is apparent that a need exists for an improved apparatus and method for controlling the temperature of a wafer during spin dispensing, which overcomes, among others, the above-discussed problems to produce a more uniform layer of process liquid over the surface of the wafer.